Bifunctional (per)fluoropolyethers (PFPEs) are known in the art for a number of applications, for example as lubricants, surfactants, coating agents or as macromonomers in polycondensation reactions. In all such applications it is desirable to use bifunctional PFPEs having a purity, or difunctional content, as high as possible, typically of at least 1.985. For example, in polycondensation reactions, the use PFPEs with a very high bifunctional content is necessary in order to avoid stochastic interruption of the polymer chain, which is observed when relevant amounts of monofunctional species are present.
However, bifunctional PFPEs are usually available in admixture with their non functional and monofunctional species, due to the fact that the synthetic methods currently available for manufacturing them do not allow to obtain pure bifunctional species. Therefore, subsequent purification is required in order to separate the difunctional species from the non functional or monofunctional ones, or to enrich the content of difunctional species. For this purpose, a number of purification processes have been developed. For instance, U.S. Pat. No. 5,246,588 (SOLVAY SOLEXIS S.P.A.) and U.S. Pat. No. 5,262,057 (AUSIMONT S.P.A.) disclose chromatographic processes, while U.S. Pat. No. 7,288,682 (SOLVAY SOLEXIS S.P.A.) and EP 0822216 A (AUSIMONT SPA) disclose batch or semi-batch processes: all of them envisage the use of a solid adsorbent stationary phase and an eluent usually comprising or consisting of a halogenated solvent, typically a fluorinated solvent. It is however well known that halogenated solvents have a negative environmental impact; thus, alternative, more environment-friendly methods based on the use of supercritical fluids, primarily supercritical (scCO2), have been proposed. For example, SCHONEMANN, H., et al. Tailoring performance properties of perfluoro-polyethers via supercritical fluid fractionation. Proc. 3rd. Int. Symp. on Supercritical fluids. 17-19 Oct. 1994, vol. 3, p. 375-380. discloses a method for the separation of PFPEs mixtures wherein scCO2 is used in the absence of a stationary phase. However, such a method only allows to separate the PFPE species on the basis of their solubility in the supercritical fluid; as a result, the PFPE species are separated according to their average numerical molecular weight (Mn), not according to their average functionality.
JP 2001164279 (HOYA CORP.) relates to a method for manufacturing functional fluorine-based lubricants for recording media and it teaches to separate low molecular weight and non functional fractions from functional fractions by supercritical fluid chromatography (SFC) on silica gel as stationary phase, using scCO2 as mobile phase. As functional fluorine lubricants, functional PFPEs bearing end groups like piperonyl, hydroxy, ester and amino groups are mentioned. According to the description and the examples, after adsorbing a PFPE mixture on the stationary phase, elution with the supercritical fluid is accomplished by adjusting temperature and pressure. This prior art further teaches that polar or non polar co-solvents can be added to adjust the solubility of the fluorine lubricant in the supercritical fluid; however, the only example of separation therein reported, which relates to the purification of a functional PFPE mixture having piperonyl end groups (Fomblin® AM-2001 PFPE), is carried out at constant pressure and temperature, without using any polar co-solvent.
SFC is mentioned also in US 2004092406 (FUJI ELECTRIC HOLDINGS CO LTD) as a method for separating PFPE compounds having low polarity terminal groups from those containing polar terminal groups; in particular, also this document refers to the separation of a highly functionalised fraction of a PFPE having piperonyl end groups (Fomblin® AM3001 PFPE) from low-polarity fractions using scCO2 at constant temperature and pressure as mobile phase and silica gel as stationary phase.
However, according to the Applicant's observations, when trying to increase the difunctional content of PFPEs mixtures, in particular of PFPE diols mixtures using scCO2 only as mobile phase and silica gel as stationary phase, for example as taught in JP 2001164279 (HOYA CORP.) and US 2004092406 (FUJI ELECTRIC HOLDINGS CO LTD), considerably long elution times, which are not convenient on an industrial scale, are necessary; instead, when only scCO2 in admixture with a polar solvent, namely methanol, is used as mobile phase, no separation or enrichment at all is achieved, because the PFPE mixture is not adsorbed on the stationary phase.
US 2003100454 (FUJI ELECTRIC CO) discloses a process for increasing the difunctional content in a mixture of mono- and bi-functional perfluoropolyether by extraction with scCO2. This document does not suggest a process comprising a first desorption step with scCO2 and a second desorption step with a mixture of scCO2 and polar solvents.
U.S. Pat. No. 6,099,937 (SEAGATE TECHNOLOGY INC) discloses a PFPE lubricant, including Fomblin® lubricants, for magnetic discs having a narrow and high molecular weight. This patent teaches that the lubricant can be obtained by various fractionation techniques, including SFC, but no specific teaching is given with regard to this method.
WO 2010/130628 (SOLVAY SOLEXIS S.P.A.) relates to a process for the purification of a polyol PFPE derivative (P) containing at least one end group of formula —CF2CH2OCH2CH(OH)CH2OCH2CH(OH)CH2OH (t3), which comprises the derivatization as ketal/acetal of a polyol PFPEs mixture containing polyol (P), submitting the derivatized mixture to chromatography on silica gel, recovering and distilling the eluate and hydrolysing the distillation residue. This document is silent on the purification of mixtures of mono- and bi-functional PFPEs and does not give any hint or suggestion to SFC separation.
There is therefore the need for an improved purification process for increasing the content of bifunctional PFPEs in mixtures of non functional, mono- and bifunctional PFPEs.